1. Field of the Invention
The present invention relates to a semiconductor laser module and a method for forming the semiconductor laser module, and in particular, to a semiconductor laser module which converts an original center wavelength of stimulated emission of a semiconductor laser to emit a laser beam of a desired wavelength and a method for forming the semiconductor laser module.
2. Description of the Related Art
A method for converting a fundamental wave to a second harmonic by using an optical wavelength conversion element, in which a region where the spontaneous polarization (i.e., domain) of ferroelectrics having a non-linear optical effect is periodically inverted is provided, has been proposed by Bleombergen et al. (see Phys. Rev., Vol. 127, No. 6, 1918 (1962)). In accordance with this method, by setting the period xcex9 of the domain inversion portion so as to be an integer multiple of the coherent length xcex9c defined by the following formula, phase matching (so-called pseudo phase matching) between the fundamental wave and the second harmonic can be carried out.
xcex9c=2n/{xcex2(2xcfx89)xe2x88x922xcex2(xcfx89)}
In this equation, xcex2(2xcfx89) is the propagation constant of the second harmonic and xcex2(xcfx89) is the propagation constant of the fundamental wave. Attempts have been made to carry out phase matching efficiently by forming such a periodic domain inversion structure in an optical waveguide type optical wavelength conversion element which converts the wavelength of the fundamental wave which propagates in an optical waveguide made of a non-linear optical material.
As one example, a conventional optical waveguide type optical wavelength conversion element, in which a periodic domain inversion structure is formed, is such that, as shown in FIG. 12, a spontaneous polarization direction of a substrate 2 shown by the arrow P is perpendicular to one substrate surface 2a (i.e., the substrate surface along which an optical waveguide 1 extends) (see Japanese Patent Application Laid-Open (JP-A) No. 5-29207).
In the above-described art, in order to carry out wavelength conversion efficiently, a Z-cut substrate of LiNbO3, LiTaO3 or the like is used as an optical element in art which uses a Z-cut substrate. In a voltage application method, when the periodic domain inversion region is formed, the inversion region tends to extend along the Z axis, and therefore, when using the Z-cut substrate, a deep periodic domain inversion structure can be formed. Accordingly, the periodic domain inversion structure is formed in the optical element and the optical waveguide is formed in a region where the periodic domain inversion structure is formed, and therefore, the wavelength conversion can be carried out with high efficiency.
In this optical wavelength conversion element, the domain inversion portion can be formed sufficiently deep from the substrate surface. However, in a case of using the optical wavelength conversion element with a semiconductor laser, an incident optical system of the fundamental wave becomes complicated. Namely, in the structure shown in FIG. 12, a beam pattern of the guided light is such that, as shown in the circle A in FIG. 12, a beam diameter in a direction parallel to the direction of the polarization vector shown by the arrow R is small and a beam diameter in a direction perpendicular to the direction is large. At this time, the direction of the polarization vector coincides with the spontaneous polarization direction of the substrate 2 (generally, in ferroelectrics such as LiNbO3 or the like, the spontaneous polarization direction is parallel to the Z axis), and the guided mode is the TM mode. On the other hand, a beam pattern of a laser beam 4 emitted from a semiconductor laser 3 is such that, as shown in the circle B in FIG. 12, a beam diameter in the direction parallel to the direction of the polarization vector shown by the arrow Q is large and a beam diameter in a direction perpendicular to the direction is small. When respective polarization directions are aligned in order to input the laser beam 4 emitted from the semiconductor laser 3 into the optical waveguide 1, beam shapes are mismatched and therefore, the laser beam 4 cannot be entered efficiently into the optical waveguide 1. Accordingly, the intensity of the second harmonic is small.
In order to rotate the polarization direction of the laser beam 4 90 degrees without changing the beam pattern, a complicated fundamental wave incident optical system is needed, in which a xcex/2 plate 7 is disposed between a collimator lens 5 and a condenser lens 6.
To solve the above-described problem, an optical element, in which an optical wavelength conversion element and a semiconductor laser are integrated, has been proposed (see Japanese National Publication No. 10-506724). In this art, the semiconductor laser, to which a wavelength tuning mechanism is applied, is directly mounted to the optical wavelength conversion element (waveguide SHG). Since the optical wavelength conversion element is excited by the semiconductor laser to which the wavelength tuning mechanism is applied, a center wavelength of stimulated emission of the semiconductor laser is made to coincide with a phase matching wavelength of the optical wavelength conversion element.
However, the semiconductor laser emits semiconductor laser light in a TE mode in which the polarization direction of the semiconductor laser light is parallel to a substrate. In contrast, in the optical wavelength conversion element, semiconductor laser light is emitted in a TM mode in which the polarization direction of the semiconductor laser light is perpendicular to the substrate. Thus, as mentioned above, in order to mount the semiconductor laser, to which the wavelength tuning mechanism is applied, directly to the optical wavelength conversion element, the polarization directions must be made to coincide with each other, and the substrate of the semiconductor laser and the substrate of the optical wavelength conversion element must be mounted so as to be perpendicular to each other. In the conventional optical element, alignment must be carried out with high accuracy in order to make the polarization directions coincide with each other.
In view of the aforementioned, an object of the present invention is to provide a semiconductor laser module by which high wavelength conversion efficiency can be obtained easily, and a method for forming the semiconductor laser module.
In order to accomplish the object, there is provided a semiconductor laser module comprising: an optical wavelength conversion element which is formed such that, on a ferroelectric crystal substrate having a non-linear optical effect, a TE mode optical waveguide which extends along a substrate surface and in which a polarization direction is parallel to the substrate is formed, and a domain inversion portion, where a spontaneous polarization direction of the substrate is inverted, is periodically formed in the optical waveguide, and the optical wavelength conversion element converts a wavelength of a fundamental wave which propagates in the optical waveguide in a direction along which the domain inversion portions are aligned; and a semiconductor laser which can emit a laser beam in the TE mode in which a polarization direction is parallel to the substrate and which can adjust a center wavelength of stimulated emission of the laser beam, and light emitted from the semiconductor laser is made to enter the optical waveguide, wherein the optical wavelength conversion element and the semiconductor laser are mounted such that the polarization directions of the TE mode coincide with each other and a light exit portion of the semiconductor laser and a light entrance portion of the optical wavelength conversion element coincide with each other.
As described above, because both the optical wavelength conversion element and the semiconductor laser can propagate light in the TE mode in which the polarization direction of laser light is parallel to the substrate, the substrates of the optical wavelength conversion element and the semiconductor laser can be disposed parallel to one another. Accordingly, it is possible to form the semiconductor laser module easily without disposing the optical wavelength conversion element and the semiconductor laser perpendicular to one another and joining them. Since the position of the exit portion of the semiconductor laser and the position of the entrance portion of the optical wavelength conversion element are made to coincide with each other, it becomes easy for the semiconductor laser and the optical wavelength conversion element to receive and output lights, i.e., laser beams, that is to say, it is easy for the optical wavelength conversion element to receive the light, i.e., laser beam, emitted from the semiconductor laser.
In the semiconductor laser module of the present invention, the semiconductor laser can emit laser beams in the TE mode and the center wavelength of stimulated emission of the laser beam can be adjusted. Examples of structures in which the center wavelength of stimulated emission can be adjusted include Distributed Bragg Reflector and Distributed Feedback. With these structures, for example, the center wavelength of stimulated emission of the semiconductor laser can be easily adjusted to the phase matching wavelength of the optical wavelength conversion element. Accordingly, it is possible to adjust the center wavelength of stimulated emission of the semiconductor laser to a wavelength at which the efficiency of the optical wavelength conversion is maximized, and to make the output quantity of light a maximum.
In the optical wavelength conversion element of the semiconductor laser module of the present invention, the spontaneous polarization direction of the substrate forms a predetermined angle xcex8 with respect to the substrate surface within a plane perpendicular to the propagation direction of the fundamental wave. The angle xcex8 is preferably set so as to be greater than 0.2xc2x0. In a case of forming the optical waveguide by proton exchange and annealing, the angle xcex8 is preferably set so as to be greater than 0.5xc2x0.
In this way, the spontaneous polarization direction of the substrate, i.e., the Z axis direction, forms a predetermined angle xcex8 with respect to the substrate surface, and thus, the spontaneous polarization direction of the substrate, i.e., the Z axis direction, is not perpendicular to the substrate surface. Thus, even if the laser beam which exits from the semiconductor laser enters into the optical waveguide with its linear polarization direction being parallel to the substrate surface, the non-linear optical constant d33 is used and the wavelength can be converted.
It is known that if an angle xcfx86 formed by the Z axis and the substrate surface is 0xc2x0 less than xcfx86 less than 20xc2x0, a light beam propagates in the TE single mode in an optical waveguide which is formed by proton exchange and subsequent annealing. Accordingly, when the optical waveguide is formed by the proton exchange and annealing, the angle xcex8 is set to xcex8 less than 20xc2x0, so that wavelength conversion can be carried out efficiently.
On the other hand, when a domain inversion structure with an optimal duty ratio, by which maximum wavelength conversion efficiency is obtained, is formed, a cycle for forming the domain inversion portion is generally 50 xcexcm, provided that the angle xcex8 (see FIG. 4) is within a few degrees. Generally, a field distribution of the guided mode can be 1.2 xcexcm at its thinnest. Accordingly, if xcex8=0.2xc2x0, the depth of the domain inversion portion is 1.2 xcexcm, and the domain inversion portion is almost the same size as the field distribution of the guided mode in the depthwise direction. If 0.2xc2x0 less than xcex8, the domain inversion portion sufficiently overlaps the field distribution of the guided mode, and therefore, wavelength conversion can be carried out efficiently.
The semiconductor laser module can be manufactured easily by the following method for forming the semiconductor laser module. Specifically, the method for forming a semiconductor laser module comprises the steps of: forming an optical wavelength conversion element which is formed such that, on a ferroelectric crystal substrate having a non-linear optical effect, a TE mode optical waveguide which extends along a substrate surface and in which a polarization direction is parallel to the substrate is formed, and a domain inversion portion, where a spontaneous polarization direction of the substrate is inverted, is periodically formed in the optical waveguide, and the optical wavelength conversion element converts a wavelength of a fundamental wave which propagates in the optical waveguide in a direction along which the domain inversion portions are aligned, wherein in a plane perpendicular to a propagation direction of the fundamental wave, the spontaneous polarization direction of the substrate forms a predetermined angle with respect to the substrate surface; forming a semiconductor laser which can emit a laser beam in the TE mode in which a polarization direction is parallel to the substrate, and which can adjust a center wavelength of stimulated emission of the laser beam, and light emitted from the semiconductor laser is made to enter the optical waveguide; and mounting the formed optical wavelength conversion element and the formed semiconductor laser such that the polarization directions of the TE mode coincide with each other and a light exit portion of the semiconductor laser and a light entrance portion of the optical wavelength conversion element are made to coincide with each other.
The method for forming a semiconductor laser module further comprises the steps of: forming a substrate for fixing on which the optical wavelength conversion element and the semiconductor laser are mounted, the substrate for fixing having a flat surface and a stepped surface with a predetermined step which is parallel to the plane; and mounting the optical wavelength conversion element to the flat surface of the substrate for fixing, and mounting the semiconductor laser to the stepped surface of the substrate for fixing.
As a result, alignment in a direction intersecting the flat surface, i.e., in the vertical direction, is not necessary, and the optical adjustment axis is only along the horizontal direction. Therefore, adjustment is easy and misalignment after fixing can be suppressed. It is preferable that the upper surface of the semiconductor laser is joined to the substrate for fixing, and the optical wavelength conversion element is adhered to the stepped surface of the substrate for fixing. In this case, the waveguide faces downward, i.e., positioned further toward the lower side of the stepped surface and it is difficult to effect adjustment while observing from above by using a magnifier such as microscope or the like. However, the substrate of the optical wavelength conversion element is generally transparent, so it is possible to observe easily even from above.
The method for forming a semiconductor laser module further comprises the steps of: forming an optical wavelength conversion element holder which has a reference surface for light entry and is able to fix the optical wavelength conversion element such that a plane of light entry of the optical wavelength conversion element includes the reference surface for light entry; forming a semiconductor laser holder which has a reference surface for light exiting and is able to fix the semiconductor laser such that a light exiting surface of the semiconductor laser includes the reference surface for light exiting; fixing the optical wavelength conversion element to the optical wavelength conversion element holder, and fixing the semiconductor laser to the semiconductor laser holder; and mounting the optical wavelength conversion element and the semiconductor laser such that the reference surface for light entry of the optical wavelength conversion element holder and the reference surface for light exiting of the semiconductor laser holder are joined.
In the holders, end surfaces of respective elements can be arranged, for example, in a straight line. The optical wavelength conversion element and the semiconductor laser can be mounted easily by setting the holders to oppose one another such that the sides thereof with the aligned end surfaces face each other. The position of the exit of the laser beam of the semiconductor laser and the position of the entrance to the waveguide of the optical wavelength conversion element can be adjusted in a planar manner. Accordingly, the semiconductor laser and the optical wavelength conversion element can be easily fixed together while the bonding efficiency is being observed.